[TD] Use isProjectGroupItem() in remaining cases

- isDerivedFrom<DrawProjGroupItem>() is no longer reliable due to smart
  view creation changes.

src/Mod/TechDraw/Gui/ViewProviderViewClip.cpp

@@ -35,6 +35,7 @@
 #include "QGIViewClip.h"
 
 using namespace TechDrawGui;
+using namespace TechDraw;
 
 PROPERTY_SOURCE(TechDrawGui::ViewProviderViewClip, TechDrawGui::ViewProviderDrawingView)
 
@@ -123,7 +124,8 @@ void ViewProviderViewClip::dragObject(App::DocumentObject* docObj)
 
 void ViewProviderViewClip::dropObject(App::DocumentObject* docObj)
 {
-    if (docObj->isDerivedFrom<TechDraw::DrawProjGroupItem>()) {
+    auto dvp = freecad_cast<DrawViewPart*>(docObj);
+    if (dvp && DrawView::isProjGroupItem(dvp)) {
         //DPGI can not be dropped onto the Page if it belongs to DPG
         auto* dpgi = static_cast<TechDraw::DrawProjGroupItem*>(docObj);
         if (dpgi->getPGroup()) {